A conventional series resonant inverter power supply circuit for an induction furnace, or other loads with inductive impedance, includes a DC power source and a plurality of inductive reactors in series with inverter power switching devices, such as silicon controlled rectifiers (SCRs). The inductive reactors limit the rate of change of current in the SCRs during commutation and are often referred to as "di/dt reactors." The SCRs are connected in series with an induction coil, and are triggered to be alternately conductive and non-conductive. With this arrangement, the SCRs alternately allow current to flow through the induction coil, which will flow in alternating directions. This creates an AC current in the coil.
Parallel-resonant inverter power supply circuits are also used to excite induction heating and melting coils. However, series-resonant inverter power supplies are preferred because of their higher efficiency. A problem with series-resonant inverter power supplies is their vulnerability to short circuit conditions on the induction coil. It is relatively common for molten metal spills or pieces of metal scrap being loaded into a furnace to come into contact with the introduction coil, and short out two or more coil turns. This is a serious concern, since a shorted coil can cause severe damage to the inverter power supply.
For example, in the event of a short circuit across two or more turns of the induction coil, an instantaneous and generally catastrophic overvoltage condition across the nonconducting SCRs in the inverter could occur. In the past, attempts were made to deal with such an overvoltage condition, if such a condition were detected at all, by triggering the affected SCRs into conduction in order to eliminate the overvoltage condition across the SCRs. U.S. patents which disclose inverter power supplies and protection measures of the type just described include U.S. Pat. Nos. 4,060,757, 4,570,212, 4,710,862, 5,235,487, and 5,418,706.
However, this method has its drawbacks, not the least of which is that it causes extremely high current to flow through the SCRs which, in turn, produces great amounts of heat within the SCRs in a very short period of time. The SCRs are, in effect, forced to withstand an extremely high current in order to avoid being subjected to an overvoltage. This condition can lead to severe voltage stress on the SCRs and their premature failure.
The present invention solves the problem of overvoltage-induced failure by reducing the overvoltage in the first instance. The present invention adds an inductance in series with the induction coil to suppress the overvoltage that would otherwise occur across the switching devices in the event of a short in the induction coil, thus protecting the switching devices from both severe voltage stress and thermal damage. With the present invention, it is no longer necessary to force the switching devices to absorb high currents to avoid being subjected to overvoltage conditions.